The average power output of typical microwave power devices is limited by the inability of such devices to adequately dissipate heat which is generated by the high currents within such devices. For example, in a typical prior art traveling wave tube, a metal helix is concentrically supported within an elongated, evacuated ceramic or metal cylinder. The helix is usually supported by dielectric rods which maintain alignment of the helix within the cylinder and serve to transfer heat to the cylinder walls and thence to the outside environment. The principal heat transfer mechanism is by conduction through the dielectric supports. However, even with the use of exotic dielectric materials with large heat conductivity, such as diamond or beryllia (beryllium oxide), there are limitations upon the amount of heat that may be carried away from the helix. Such limitations, of course, place restrictions upon operation of the tube--particularly upon the amount of current which the helix can tolerate. Furthermore, it is impossible to obtain perfect contact at the helix-rod and rod-cylinder interfaces. Prior art attempts at improving contact at these interfaces involve metallizing the helix-rod interface together with heat shrinking techniques to press-fit the rods against the inside surfaces of the cylinder. Nevertheless, heat transfer is still limited because the helix only touches a particular rod at one point per turn. Backward wave oscillators and crossed field amplifiers which also use helical slow wave circuits exhibit the same problems and have endured similar attempts at solution.
U.S. Pat. No. 4,347,419, entitled "Traveling Wave Tube Utilizing Vacuum Housing As An RF Circuit", issued to the present inventor, discloses a traveling-wave-tube with a cylinder that serves as a vacuum housing and also has an integral helical slow wave circuit. The helix conductor is intertwined with and hermetically sealed to the material which comprises the vacuum housing. Thus, heat may be readily transferred from the inside of the housing to the outside of the housing since the helix is not supported by dielectric rods but is instead an integral part of the vacuum housing. The helix is typically made from copper or other conductive material. However, although copper nd other metals such as silver or tungsten may be good or even excellent conductors of both heat and electricity, they do nevertheless exhibit demonstrable losses.
Recent developments in the field of superconductivity have produced a large variety of new ceramic-type materials which are capable of achieving the superconducting state at critical temperatures above 77.degree. K., the boiling point of liquid nitrogen. The critical temperature is the temperature at which the material becomes superconducting. The new class of materials (termed for convenience, "superconducting ceramics" herein--even for materials which are not basically ceramic in nature) has been extensively discussed in the popular press. For example, the New York Times, on Mar. 20, 1987 reported the existence of superconducting ceramics and described the making of such materials into sheets of vinyl-like tape and washer shapes. Furthermore, Electronics in its Apr. 2, 1987 issue on pp. 49-51 reported the making of superconducting ceramics into wire shapes.
The composition and manufacture of superconducting ceramics is discussed, for example, in Physics Today, pp. 17-23, April 1987, which is incorporated herein by reference. An entire class of compounds with the chemical composition is RBa.sub.2 Cu.sub.3 O.sub.9-y, where R stands for a transition metal or a rare earth ion and y is a number less than 9, preferably 2.1.+-.0.05, has demonstrated superconductive properties above 90.degree. K. This class of materials is included in the terms "superconducting ceramic" and "rare-earth doped copper oxide" as used herein. Scandium, lanthanum, neodymium, samarium, europium, gadolinium, dysprosium, holmium, erbium, ytterbium, yttrium, and lutetium are acceptable substitutes for R above. The crystal structure of these compounds is described as an orthorhombically distorted perovskite structure.
Some compounds are formulated substituting strontium for barium. For example, La.sub.2-.chi. SR.sub..chi. CUO.sub.4-.chi. has exhibited superconductivity at high temperatures, as reported in Physical Review Abstracts, p.13, vol. 18, No. 8, Apr. 15, 1987.
Fabrication of superconducting ceramics is discussed in the above-mentioned Physics Today article. A detailed discussion of the fabrication and physical properties of a typical superconducting ceramic is also found in: R. J. Cava et al., Bulk Superconductivity at 91.degree. K. in Single Phase Oxygen - Deficient Perovskite Ba.sub.2 Y Cu.sub.3 O.sub.9-.delta., Physics Review Letters, pp. 1676-1679, April 20, vol. 58, number 16.
Superconducting ceramics with high critical current densities may be (in excess of 10.sup.5 A/cm.sup.2) produced by growing epitaxial films of RBa.sub.2 Cu.sub.3 O.sub.7-.chi. on SrTiO.sub.3 substrates as taught in P. Chaudhari et al., "Critical Current Measurements in Epitaxial Films of YBa.sub.2 Cu.sub.3 O.sub.7-.chi. Compound," Phys. Rev. Ltrs. vol 58, no 25, pp. 2684-2686, 22 June 1987.
It has been further been determined that superconducting ceramics may be formed by plasma-spraying techniques, such as those described in the aforementioned U.S. Pat. No. 4,747,419.
In addition, it has been determined that semiconductor fabrication techniques such as laser ablation, electron beam epitaxy, sputtering, ion implantation and plasma spraying may be used to form superconducting ceramics.
The Asbury Park Press on May 3, 1987, reported that scientists have developed plasma spraying techniques to coat items--including tubes made from ceramic, quartz and metals of various sizes--with superconducting ceramic coatings. Plasma spraying is a technique in which a material (in this case a superconducting ceramic) is heated to a high temperature and then deposited on a cool surface where it solidifies. After coating, the objects are subsequently annealed.
Furthermore, ion implantation techniques have been developed for fabricating side-by-side layers of superconducting and non-superconducting materials.
Those concerned with the development of microwave power devices have continuously sought methods and apparatus for improving the performance of such devices and for reducing the resistive losses (and consequent heat dissipation problems) inherent in slow-wave structures. Furthermore, those concerned with the development of superconducting ceramics have engaged in a continuing search for new application for these materials.